Rubber compositions for tire sidewalls may contain various diene rubbers, including natural rubber (NR) and polybutadiene rubber (BR). However, since these rubbers have a high degree of unsaturation, i.e. they contain a high amount of carbon-carbon double bonds, they are highly susceptible to oxidation and ozone. To improve the resistance to oxidation and ozone, it is known to add antioxidants and antiozonants to the rubber composition. Despite the improved resistance to oxidation and ozone, many of the antioxidants and antiozonants migrate to the surface of the sidewall causing the sidewall to have an unattractive brown appearance.
To reduce the amount of necessary antioxidants and antiozonants, it is known to add polymers having a low degree of unsaturation which are less susceptible to oxidation and ozonation, such as ethylene propylene diene monomer (EPDM). However, polymers having a low degree of unsaturation typically have a lower affinity towards fillers such as carbon black and silica. This results in less reinforcement in the polymer, leading to lower crack growth resistance.
It is also known to add functional groups to polymers in order to increase the affinity between a polymer and filler. Functional groups may be added by functional initiators and/or functional terminators. This process yields at most two functional groups per polymer chain. However, due to the high molecular weight of polymers, there are typically relatively few functional groups available to interact with the filler.
There is a need for a method to improve the filler affinity to polymers having a low degree of unsaturation.